1. Field of the Invention
The present invention relates to a fuel cell power supply device for limiting an output current of a fuel cell and recovering the fuel cell back into a normal state when the fuel cell suffers a malfunction.
2. Description of the Related Art
There has heretofore been employed a fuel cell power supply device which uses a fuel cell stack 100 for a vehicle such as an electric vehicle or the like, as shown in FIG. 7 of the accompanying drawings. In the fuel cell power supply device shown in FIG. 7, the fuel cell stack 100 supplies electric energy through a current limiter 101 to a motor 102 and an electric accessory 103 such as an air-conditioning unit. An electric double layer capacitor 104 is connected parallel to the fuel cell stack 100 for supplementing the electric energy that is supplied to the motor 102 and the electric accessory 103.
The fuel cell stack 100 generates an amount of electric energy which is increased or reduced depending on the amount of reactive gases, i.e., a reducing gas such as hydrogen or the like and an oxidizing gas such as air or the like for extracting electric energy by reacting with the reducing gas, supplied from a reactive gas supply unit 105. A controller 106 controls the amount of reactive gases supplied from the reactive gas supply unit 105 to the fuel cell stack 100 so that the amount of electric energy required to operate the motor 102 and the electric accessory 103 will not become excessive or insufficient.
Specifically, the controller 106 determines a target output current (Ifc_CMD) of the fuel cell stack 100 depending on the amount of depression (Ap) of the accelerator pedal, the vehicle speed (Nm) of the vehicle, and the electric energy (Pload) consumed by the electric accessory 103, and controls the reactive gas supply unit 105 to supply an amount of reactive gases depending on the target output current (Ifc_CMD) to the fuel cell stack 100.
Since the controller 106 controls the amount of reactive gases supplied from the reactive gas supply unit 105 to the fuel cell stack 100, the current outputted from the fuel cell stack 100 basically remains large enough to provide the electric energy that is consumed by the motor 102 and the electric accessory 103. However, water produced in the fuel cell stack 100 when a current is generated by an electrochemical reaction of the reactive gases in the fuel cell stack 100 may possibly not be fully discharged from the fuel cell stack 100 but may be trapped in some of the cells (not shown) that make up the fuel cell stack 100.
If such water is trapped in some of the cells, then the reactive gases are not supplied to those cells, causing the cells to reduce their output currents. When the fuel cell stack 100 suffers such a malfunction, the cells whose output currents are reduced by the trapped water are likely to fail due to currents outputted from the other normal cells.
The controller 106 is designed to avoid the above drawback as follows: The controller 106 recognizes an operating state of the fuel cell stack 100 based on an output from a fuel cell sensor 107 which detects the pressure, rate, and temperature of the supplied reactive gases and individual states of the cells of the fuel cell stack 100, and determines an upper limit (Ifc_LMT) of the current that can be outputted from the fuel cell stack 100 based on the recognized operating state of the fuel cell stack 100.
When the upper-limit output current (Ifc_LMT) becomes equal to or lower than the target output current (Ifc_CMD), the controller 106 judges that the fuel cell stack 100 is malfunctioning, and controls the current limiter 101 to limit the current outputted from the fuel cell stack 100. When the current outputted from the fuel cell stack 100 is limited, the current outputted from the fuel cell stack 100 becomes short of the target output current (Ifc_CMD). At this time, the electric double layer capacitor 104 discharges a current to make up for current shortage, thus supplying a sufficient current according to the target output current (Ifc_CMD).
If the malfunctioning of the fuel cell stack 100 continues, however, the amount of electric energy stored in the electric double layer capacitor 104 progressively decreases, and hence the amount of electric energy discharged from the electric double layer capacitor 104 also progressively drops, failing to supply a sufficient current according to the target output current (Ifc_CMD).